Use of lead free-alloys for C4 balls is increasing to reduce environmental impacts. Upon solidification, the tin-based (Sn-based) prior art alloy materials tend to form large grains in the lead-free C4 balls. The number of grains in the solidified lead-free C4 balls is correspondingly small, e.g., typically from 1 to 5, and more typically from 1 to 3. Tin has a tetragonal crystal structure and the physical properties, such as deformation under stress and diffusion, depend critically upon Sn-grain orientation. When a small number of Sn-grains are present in a typical C4 ball, undesirable Sn-grain orientation may result in dramatically increased diffusion rates, which are enhanced by the electron current and thereby cause dramatic decreases in electromigration reliability. In particular, if the solder joint is comprised of only a few grains with a single dominant grain and this grain has its, crystallographic, c-axis oriented parallel to the current direction, the time to failure by electromigration processes in dramatically reduced. Sn grains are highly anisotropic and the early electromigration failures derive from the rapid diffusion transport along the c-axis of Sn—particularly for Cu and Ni, which often comprise the solder pad materials and the associated interfacial layered intermetallic compounds.
Thus, formation of a small number of grains in the lead-free C4 ball, e.g., 1 or 2, creates more reliability concerns with respect to chip interconnect reliability than formation of a large number of grains, randomly oriented, in the lead-free C4 solder joint, all other parameters for the C4 balls being equal. Many-grained C4 balls, containing, for example, 5 or 10 grains per C4 ball with random orientation, is conducive to mitigation of the diffusive fluxes and enhanced electromigration lifetimes. However, increasing the number of grains in a C4 ball has been very difficult to achieve. For electromigration reliability, decreasing diffusion rates inside the Sn-grains and along grain boundaries is desirable and could potentially be achieved with suitable Sn-alloy compositions.
In addition it has also been demonstrated in the general literature that solder joints comprised of a single grain or only a few grains are disposed to early failure by thermomechanical fatigue process. Solder joints with just a few additional grains demonstrate significantly increase thermomechanical fatigue life. The worst Sn grain configuration for thermomechanical fatigue in many solder joint application is for the solder joint to be comprised of only a single grain and having the c-axis of this grain oriented parallel to the solder pad surfaces.
In combination these facts imply that the best Sn grain crystallographic orientations for that associated electromigration failure and that for fatigue failure are orthogonal. However, for both failure modes Sn grains comprised of several grains show significant improvement in failure rates for both electromigration and for thermomechanical fatigue.